Conventional vibrating type screens are used almost exclusively with a vertically oriented action, small in dimension but applied at fairly high speed. Such vibration has been necessary to process small particles and is normally used in combination with wash water if the apparatus is used in an environment such as the coal industry. However, the high frequency, small throw motion is not effective for moving large volumes of granular material and the vertical action in itself does not provide high capacity screening.
Some attempts have been made to apply a compound vibration to a screen as exemplified in U.S. Pat. No. 3,378,142 of Apr. 16, 1968 to WEHNER and U.S. Pat. No. 3,261,469 of July 19, 1966. These patents disclose a vibratory screen device in which a compound vibration of the screen is disclosed. These motions are a vertical and a circular or semi-circular to-and-fro motion to the screen, these motions being applied through the use of a single motor. However, these patents do not disclose an apparatus similar to the present invention where two distinct and separate motions are applied to the screen for greater control over the action of the apparatus.